DIFFERENTIAL SECRETION OF FOLLICLE-STIMULATING-HORMONE AND LUTEINIZING-HORMONE FROM OVINE PITUITARY-CELLS FOLLOWING ACTIVATION OF PROTEIN-KINASE-A, PROTEIN-KINASE-C, OR INCREASED INTRACELLULAR CALCIUM

A higher dose of GnRH is required to stimulate release of FSH than of LH, both in vivo and in vitro. Therefore, we tested the hypothesis tha t secretion of FSH may be mediated via a second messenger pathway diff erent from the one that modulates secretion of LH. Pituitary cells fro m intact ewes were cultured in suspension in DMEM plus 1O% wether seru m. After 18 h, cell were washed and challenged for 2 h with agents cap able of activating protein kinase A (dibutyryl cAMP), protein kinase C (phorbol 12-myristate 13-acetate; PMA), or increasing intracellular c alcium (the calcium ionophore A23187). GnRH (0.01-10 nM) and PMA (0.2- 20 nM) stimulated dose-dependent increases in secretion of LH. FSH sec retion also was stimulated by GnRH and PMA; however, the percentage of total cellular FSH released was lower (p < 0.05) than the percentage of total cellular LH released. Dibutyryl cAMP (10 mM) induced a modest release (p < 0.05) of both LH and FSH. A23187 (1-10 mu M) stimulated secretion of LH in a dose-dependent manner but did not influence secre tion of FSH; however, GnRH- and PMA-induced secretion of FSH required the presence of intracellular calcium. On the basis of the results of this study, we suggest that secretion of FSH is less than secretion of LH following direct activation of these second messenger systems. Fur thermore, we suggest that in contrast to the situation for LH, increas ed intracellular calcium is not the primary stimulus for inducing secr etion of FSH.